Earlier today, Apple posted an online database where prospective iPhone 3GS buyers can check the availability status of the iPhone 3GS in their region.
Keep in mind that this database only shows retail outlets where you are guaranteed (or not guaranteed) to get an iPhone 3GS. At present, only 5 of Apple’s 200 retail stores are completely sold out of the iPhone 3GS.
Apple did something almost exactly like this last year when the iPhone 3G first went on sale.
The database is updated hourly and can be found here

Sat, Jun 27, 2009 | by Tim Bradbury
2nd gen iPhone, Apple Inc., Apple Stores, iPhone News